Optically variable pigments have been described in the patent literature since the 1960s. Hanke in U.S. Pat. No. 3,438,796 describes the pigment as being xe2x80x9cthin, adherent, translucent, light transmitting films or layers of metallic aluminum, each separated by a thin, translucent film of silica, which are successively deposited under controlled conditions in controlled, selective thickness on central aluminum film or substrate.xe2x80x9d These materials are recognized as providing unique color travel and optical color effects.
The prior art approaches to optically variable pigments have generally adopted one of two techniques. In the first, a stack of layers is provided on a temporary substrate which is often a flexible web. The layers are generally made up of aluminum and MgF2. The stack of film is separated from the substrate and subdivided through powder processing into appropriately dimensioned particles. The pigments are produced by physical techniques such as physical vapor deposition onto the substrate, separation from the substrate and subsequent comminution. In the pigments obtained in this way, the central layer and all other layers in the stack are not completely enclosed by the other layers. The layered structure is visible at the faces formed by the process of comminution.
In the other approach, a platelet shaped opaque metallic substrate is coated or encapsulated with successive layers of selectively absorbing metal oxides and non-selectively absorbing layers of carbon, metal and/or metal oxide. To obtain satisfactory materials using this approach, the layers are typically applied by chemical vapor deposition techniques in a fluidized bed. A major shortcoming of this technique is that fluidized bed processes are cumbersome and require substantial technical infrastructure for production. An additional limitation related to the substrates utilized is that traditional metal flakes usually have structural integrity problems, hydrogen outgassing problems and other pyrophoric concerns.
The prior art approaches suffer from additional disadvantages. For instance, certain metals or metal flake such as chromium and aluminum, specifically when they are used as outer layers may have perceived health and environmental impacts associated with their use. The minimization of their use in optical effect materials should be advantageous due to their perceived impact.
The present invention provides a color effect material comprising a platelet-shaped substrate encapsulated with (a) a first layer highly reflective to light directed thereon; (b) a visibly transparent second organic layer encapsulating the first layer and providing a variable pathlength for light dependent on the angle of incidence of light impinging thereon in accordance with Snell""s Law; and (c). a selectively transparent third layer to light directed thereon.